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Caring for indoor plants in the wintertime

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Nerve plant.

Photo: Jardin botanique de Montréal (Gilles Murray)

Winter is a difficult season for many indoor plants because they are ill adapted to the constant heat, dry air and lack of sunlight in our homes. Let’s take a closer look at how to diagnose the symptoms that can result, and some possible solutions.

Let there be light!

Plants must have sunlight in order to grow properly. It gives them the energy they need to form lush foliage and good blooms.

From November to February, as the days get shorter and light levels decline, your plants may show signs of suffering from a lack of sunlight. Their leaves may be farther apart and faded, and new leaves may be small and yellowish, while plants with variegated foliage may gradually lose their colour. As soon as your plants start to show any of these signs, lower the room temperature and try to give them as much sunlight as possible. Raise the blinds or open the drapes.

When the days begin to lengthen and the sunshine becomes stronger, in March, it is time to filter the sunlight with blinds or drapes or to move your plants farther away from the windows. Too much light will cause the foliage to gradually fade, and brownish spots to occur. Rotate your pots regularly to help your plants grow more uniformly, since they will tend to grow toward the source of light.

You can supplement insufficient natural light with an artificial light source. The most important wavelengths for photosynthesis are blue and red. Red (660 nm) and far red (735 nm) will encourage stems to lengthen, while blue light (435 nm) stimulates leaf development.

Incandescent bulbs or projectors are not a good idea. They use too much energy and produce too much heat, and if placed too close to your plants could burn the foliage and dry the soil out too quickly. It is better to use them as supplementary lighting to create special effects.

Fluorescent tubes are much better: they give off little heat, produce more light per watt and can be placed very close to your plants (30 to 45 cm). “Cool white” and “day light” tubes produce a lot of blue light, but little red, whereas “warm white” tubes produce much more red than blue. You can try to cover the entire spectrum by using one tube of each type in a double fixture. Tubes designed especially for plants, or “grow lights,” are easy to use and produce the full spectrum needed for plant growth, but are expensive. The amount of light produced by fluorescent tubes gradually decreases over time, so they should be replaced every year.

Mercury, halogen and sodium lights are a good light source, but they are prohibitively expensive for most home gardeners.

As a rule, foliage plants do well with 12 to 14; hours of light a day, while many flowering plants (except those that prefer shorter days, like chrysanthemums and poinsettias) need 14 to 15; hours of light to initiate blooming.

Keep an eye on temperatures and humidity

The quality of the ambient air affects plant growth. Indoor heating in winter causes the air to become hot and dry. Leaf tips and edges tend to become dry and brown and there is an increased risk of spider mites and other insect pests. Avoid placing your plants too close to heat sources and keep temperatures on the cool side. Most plants appreciate having the thermostat turned down by 3°C at night. Keep your plants out of drafts from doors and windows, too.

There are various ways to increase the relative humidity: you can use a humidifier, group your plants together but not touching or stand the pot on large a saucer filled with pebbles and water. Make sure not to let the base of the pot stand in the water, or the roots could rot. You can also mist your plants’ foliage with water. Most plants enjoy 40 to 60% relative humidity.

Cut back on watering and feeding

Pay careful attention to watering. Let tap water stand for a few hours to let the chlorine evaporate and for it to come to room temperature. Avoid watering little and often – it is best to wait until the soil surface is slightly dry and then water deeply. While watering, cover the entire soil surface until water starts to run out of the drainage holes. Pour off any water standing in the saucer. Over-frequent watering will leach the soil nutrients, compact the soil structure and reduce the air spaces in the mix, asphyxiating the plant.

Don’t expect there to be any hard and fast rules about how frequently to water your plants. Rather, it depends on each plant’s stage of growth and its species, the size and type of pot, and the room temperature and relative humidity. A growing medium containing a lot of peat moss will be more moisture retentive than one with more loam or sand, but more difficult to rehydrate if it dries dry out. Plants in plastic pots and cachepots need to be watered less frequently than ones in clay pots. You’ll also need to adjust your watering schedule to the air in your home: dry, overheated air in an apartment will mean more frequent watering. As a rule, plants need to be watered and fertilized more frequently from March to October, during their active growing period, and you should gradually cut back on watering and feeding from October to February, when they are growing most slowly.

Many growers even recommend holding off on feeding entirely between October and February. Some plants require a rest period, when you should cut watering back to an absolute minimum. Careful though!

Never allow the soil or the plant to dry out completely. If your plant is suffering from a lack of water, it will wilt and the lower leaves will turn yellow. If that happens, just add some water. If it is difficult to rehydrate the soil, you can soak the pot in water for several minutes, or until no more air bubbles appear. Then resume watering at normal intervals. You can tell if a plant is suffering from over-watering if the stems and leaves turn soft and small brown spots appear. To help dry out over-watered soil, you can place the pot or the rootball (after removing it from the pot) on newspaper, which will absorb the excess moisture. Then return the pot to its saucer and allow the soil to dry out to the point that is right for that particular plant before resuming regular watering.

The following plants tend to thrive as indoor plants, with the proper care: